Throughout this description and the claims which follow, unless the context requires otherwise, the word “comprise’, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps.
The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in Australia.
Paper consists of a complex web of cellulosic fibres more or less bonded together in a matrix. A characteristic of the fibres and the nature of the matrix holding them together creates the ability to wick and retain liquids within the matrix in such a way that the bulk of the paper becomes wetted some considerable distance from the point of entry of the liquid. This characteristic is undesirable in many applications of paper. Writing and printing paper, for example, needs to have a very low wicking characteristic in order to prevent ink from marking the fibres at any distance from its application thereby retaining clearly and sharply defined lines in only the places where the pen or printing machine has applied the ink.
However, there are other applications wherein the wicking, retaining and replacing of liquid capability of paper is a highly desirable characteristic. An example of such an application is the evaporative media used as evaporative pads in an evaporative air cooler. In this application, the cooling performance of the evaporative cooler is highly dependant on water being distributed throughout the entire matrix of the evaporative pad thereby ensuring that all of the air passing through the evaporative pad comes in contact with only wet surfaces within the pad. Cooling of the air passing through the pad by evaporation of water wetting the pad can only take place on continuously wetted surfaces. Any surface, which is not held in a continuously wet state, will not cool the air impinging on it thereby degrading the overall cooling effect of the cooler.
It is always an objective in the design of an evaporative cooler to distribute water evenly and uniformly throughout the evaporative pad in order to achieve maximum cooling effect on the air passing through. In practical terms, a completely uniform distribution of water is not possible and in any practical device there will always be parts of the evaporative pad not directly wetted. It is always highly desirable that the material from which the evaporative pad is manufactured is able to wick water from the point of application of the water to the general surface area of the material. To meet this objective, paper manufacturers offer paper which is very highly absorbent and capable of wicking liquids over a considerable distance. A related property of such paper is the capability of replacing liquid within the paper matrix. These papers are marketed as “absorbent kraft paper” or similar.
Papers used in the manufacture of such components as evaporative cooler pads must also achieve a long service life under the arduous condition of being continually wetted while in service, and subject to repeated wet and dry cycles. The paper must also withstand the rigours of exposure to sunlight (in particular the ultra-violet light component of sunlight) and extremes of climate. If the evaporative pads, or similar applications, were simply made from the paper as supplied by the paper manufacturer, the service life would be very short before the base paper material disintegrated back to a pulp.
To enhance the service life of these products, the paper is generally treated with a protecting resin which has the effect of protecting the cellulose fibres by encapsulation. It is important to achieve this encapsulation without interfering with the absorbent properties of the porous cellulose fibres. This protection process protects the cellulose from the elements and enhances the physical properties of the final product by making it stronger, more rigid and resistant to long term immersion in water. The resin is required to enhance these properties without degrading the desirable property of the ability to wick liquids. One resin used extensively for this purpose is the thermoset polymer phenyl formaldehyde, although other thermoset polymers may be used. This resin can be applied in a liquid form thereby soaking into the entire matrix of the paper and coating the cellulose fibres therein. The paper can then be processed into any desirable shape, and the resin cured and set rigid by the application of heat. Once cured, the resin surrounds and protects the fibres in the paper thereby allowing the paper to withstand long term immersion in water without degradation of physical properties. The quantity of resin incorporated into the paper must be kept within narrow confines. If too little resin content is present in the paper it is not sufficiently protected. If too much resin, the wicking characteristics of the paper are compromised.
While these methods of treating and processing paper are well known, it is found by practical example that another important characteristic of paper is not enhanced by the treatment with thermoset resin. It is found that the tendency of the paper to burn when subjected to flame or embers is still high when treated within the desired range of resin content, even though most thermoset resins are inherently non-flammable. The retained ability to burn is a by-product of the maintained porosity of the paper with open areas for water transfer. The tendency to burn is most important in some applications of treated papers. In the example above of an evaporative air cooler, an ember lodging within the evaporative pad while the cooler is not in use, and therefore dry, could easily result in the cooler catching fire causing risk to life and property.
It is highly desirable to make the paper treated for use in these applications resistant to fire to avoid these consequences. While there are many well known methods of making paper fire resistant, the arduous operating conditions of the evaporative cooler pad used as an example herein render these methods ineffective. The fire retardant mechanism used must be able to withstand continuous immersion in water for many years without leaching out and thereby becoming ineffective. It must also be non-volatile and not simply dissipate from the paper base in time. The mechanism must also not interfere with the desirable characteristics of wetability and processability of the paper.
One mechanism of rendering paper fire resistant is described by Robinson et al in U.S. Pat. No. 5,723,020 “Fire-retardant saturating kraft paper”. The method described by Robinson adds the fire retardant chemicals alumina trihydrate and sodium borate to the paper's cellulose structured web during the manufacturing phase of the paper. While this is an effective method of fire retarding the paper, the method is only available to the manufacturer of the paper since the structure of the cellulose structured web can only be enhanced prior to the processing of the paper pulp into paper. That method is of no use to a manufacturer buying absorbent kraft paper from the general market for conversion into, for example, evaporative pads for evaporative coolers. Furthermore, that method results in paper which no longer has the wicking characteristic required in the application examples cited due to the filling effect on paper porosity of alumina trihydrate.
Lowe and Cabello in GB 901,663 describe a method of manufacture of flame resistant materials for use in the manufacture of laminated plastics. Their method requires the addition of a water soluble inorganic salt to the resin mixture prior to saturation of the base fibre material, and curing of the resin. While effective as a fire retardant, this method does not have any regard to the long term immersion of the resulting product in water. Testing of fire retardants which involve the simple addition of soluble inorganic salts to resin mixtures indicate that the fire retardant chemicals are readily leached out by immersion in water and subsequently when the material is dry the resistance to fire rapidly diminishes.